Constructors

Constructors- Constructors are used to initialize the instance variables of an object. Constructors are methods. They are usually declared as public to allow any code in a program to construct new objects of the class.
Ex: Platypus p1 = new Platypus();

All constructors have the same name as the class. Constructors do not have any return data type, not even void.
A constructor may take arguments (parameters) that help define a new object. A constructor that takes no arguments is called a "no-args" constructor.

A constructor is similar to a instant method, but it differs from a method in that it never has an explicit return type, it's not inherited, and usually has different rules for scope modifiers.

Naming a Constructor- Same name as the class, first letter is capitalized -- usually a noun
Return Type- No return type, not even void
Default Constructor- If you don't define a constructor for a class, a default parameterless constructor is automatically created by the compiler
Parameterized Constructor- A constructor given specific parameters
"this" Constructor- Refers to another constructor in the same class. If used, it must be the first line of the constructor

An example code of a default Constructor and Parameterized Constructor

public class Cube1 {
 
    int length;
    int breadth;
    int height;
    public int getVolume()
    {
        return (length * breadth * height);
    }
    Cube1() <---------------------------------------- Default Constructor
    {
        length = 10;
        breadth = 10;
        height = 10;
    }
    Cube1(int l, int b, int h) <--------------------- Parameterized Constructor
    {
        length = l;
        breadth = b;
        height = h;
    }
    public static void main(String[] args)
    {
        Cube1 cubeObj1, cubeObj2;
        cubeObj1 = new Cube1();
        cubeObj2 = new Cube1(10, 20, 30);
        System.out.println("Volume of Cube1 is : " + cubeObj1.getVolume());
        System.out.println("Volume of Cube1 is : " + cubeObj2.getVolume());
    }
}
 

Constructors can be recognized by the word "new". For example, FilledRect meermans = new FilledRect(....); creates a new filled rectangle. They create objects and/or initialize data.
- Default constructors: no parameters, you give things default values